Semiconductor device processing and other arts require the etching of single crystal silicon both isotropically and anisotropically. Anisotropic etchants and etching solutions are required to preferentially etch material of specific crystallographic orientation and/or conductivity type. Anisotropic etching is commonly performed both chemically and electrochemically.
Well known anisotropic chemical etchants for single crystal silicon include aqueous solutions of potassium hydroxide (KOH), sodium hydroxide (NaOH) and lithium hydroxide (LiOH). Although these solutions are sufficient for many applications, they each have an Si/SiO.sub.2 selectivity ratio only on the order of 400:1. Further, they are not particularly clean solutions in terms of ionic contamination.
Aqueous cesium hydroxide (CsOH) solution is a relatively good single crystal silicon etchant. Cs has the largest cation size of all the alkaline metals and therefore has many good properties. CsOH solution has a relatively fast etch rate and its Si/SiO.sub.2 selectivity ratio is on the order of 3000:1. However, CsOH is very expensive and therefore, impractical.
Other well known single crystal silicon etchants include the aqueous solutions of hydrazine (H.sub.2 N-NH.sub.2), ethylenediamine/pyrocatechol (H.sub.2 N-CH.sub.2 CH.sub.2 -NH.sub.2 /C.sub.6 H.sub.4 (OH).sub.2), the latter commonly being employed electrochemically. Ethylenediamine/pyrocatechol solution is an excellent etchant in that it has a relatively fast etch rate and a Si/SiO.sub.2 selectivity ratio on the order of 4000:1. Additionally, hydrazine, ethylenediamine and ethylenediamine/pyrocatechol solutions are fairly clean etchants leaving relatively few ionic contaminants. However, these etchants are extremely hazardous and difficult to use. Hydrazine is a suspected carcinogen, ethylenediamine is extremely toxic and flammable and pyrocatechol is not environmentally acceptable. For these reasons, hydrazine, ethylenediamine and ethylenediamine/pyrocatechol solutions are extremely difficult to utilize in a manufacturing environment.
In view of the above, it would be highly desirable to have an anisotropic etching solution for single crystal silicon having a fast etch rate, a high Si/SiO.sub.2 selectivity ratio and that provides a clean etch in terms of ionic contaminants and etch quality. Additionally, it would be highly desirable to have an etching solution of these qualities which is not a severe fire or health hazard and may be readily used in a manufacturing environment.